At the present time, there is an ever-present design objective of providing improved zoom lenses having an equivalent focal length (EFL) range from a dimension below the diagonal of the image frame (defined as wide angle) of the lens to a long focus or telephoto length. In the case of the popular 35 mm cameras, the diagonal of the image frame, which measures 24 mm by 36 mm, is 43.6 mm. Most camera manufacturers and lens designers strive to provide a lens having a lower equivalent focal length of 28 mm or 35 mm extending to a range of 105 mm or above. Such range or equivalent focal lengths are easy to design in a lens if compactness of the lens is not a serious design parameter.
However, where compactness of the lens, both from the standpoint of length and barrel diameter are a consideration, the design criteria for such lens becomes much more stringent. U.S. Pat. No. 4,299,454, provides a lens design which is compact and of very good optical quality. The design of U.S. Pat. No. 4,299,454 appears in the lenses of several manufacturers. More specifically, the design of this patent is directed to a three or four lens unit lens in which the second lens unit has certain power relationships to the overall power of the lens at its equivalent focal length and further which has certain parameters which provide sufficient color correction and correction for spherical aberration as the second lens unit of large negative power moves to vary the equivalent focal length. As used herein, the term "lens unit" refers to one or more lens elements air spaced from another lens unit of one or more lens elements. A lens unit may be axially movable to vary the equivalent focal length of the lens and/or compensate for variation in the position of the image frame due to change in equivalent focal length of the lens.
Also, U.S. Pat. No. 4,256,381 discloses a wide angle to long focus zoom lens in which the aperture defining mechanism remains stationary during zooming and has very closely spaced thereto an element of small optical power. This construction further enhances the compactness of a wide angle to long focus zoom lens.
The present invention also provides a compact design for a wide angle to long focus compact zoom lens in which the design of the second lens unit provides strong overall negative optical power, together with low surface contributions of spherical aberrations which means that the second lens unit is relatively insensitive to small manufacturing and assembly errors.